Mrs Brooks was one of only 12 people allowed to adopt a retired Metropolitan Police horse in 2008, the year after two people were jailed for phone hacking at her former newspaper, the News of the World.

She passed a vetting procedure after officers from the Metropolitan Police’s Mounted Branch visited her home in the Cotswolds, where she is rumoured to have gone riding with the Prime Minister.

Although the Met routinely loans retired horses to charities and members of the public, who pay for their upkeep, one MP suggested the arrangement was more evidence of the “intensely close relationship” between the Met and NI executives.

The unlikely twist came a day after the Leveson Inquiry heard that the relationship between the Met and NI was “at best inappropriately close and at worst corrupt”.

At the time Mrs Brooks took on the horse, she was editor of The Sun, but had given evidence to a committee of MPs five years earlier admitting that the News of the World had paid policemen when she was editor of the Sunday paper between 2000 and 2003.

By the time she gave the horse, called Raisa, back to the Met she was chief executive of NI and the Met was facing calls to re-open its investigation into phone hacking following the disclosure that thousands of names of potential victims appeared in notebooks seized from the private investigator Glenn Mulcaire.

On Monday the detective leading investigations into phone hacking and corruption revealed that a mole had briefed Mrs Brooks on the original investigation into voicemail interception in 2006.

Tom Watson, the Labour MP who has been at the forefront of Parliament’s attempt to uncover wrongdoing at NI, said: “In the light of the serious revelations at the Leveson Inquiry this week, the saga of the horse may seem trivial, but there is a serious question here of who authorised the loan of the horse and whether it cost any money.

“It is yet further evidence of the intensely close relationship between executives at NI and the Metropolitan Police.”

The issue of “horsegate”, as it was dubbed on Twitter, was even raised at Downing Street’s daily briefing for lobby journalists.

Asked whether David Cameron, a friend and neighbour of Mrs Brooks, had ever ridden the horse, the Prime Minister’s spokesman said: “That is not something that I keep tabs on, which horse the Prime Minister is riding."

Lord Blair, who was the Metropolitan Police commissioner at the time Mrs Brooks took on the horse, said he was unaware of the loan.

Each year around 10 of the Met’s 120 horses retire from working life.

Mrs Brooks, who is married to the former racehorse trainer Charlie Brooks, kept the horse at her farm in Chipping Norton until 2010.

It was then found a new home in Norfolk with a serving police officer, and has since died of natural causes.

David Wilson, Mrs Brooks's spokesman, said: "It's well known by people in the horse world that the Met looks for homes for horses once they retire. Rebekah took on a horse and effectively acted as a foster parent for it for a year or so.

"It's just a way of giving a temporary home to a horse that has had a distinguished service in the Met. It went off to a retirement paddock in Norfolk once it couldn't be ridden any more."

Mr Wilson said Mrs Brooks had ridden the horse, though the Met’s website says it is looking for retirement homes “where the horse will not be ridden”. A Met spokesman was unable to explain the apparent discrepancy.

A spokesman for Scotland Yard said: "When a police horse reaches the end of its working life, Mounted Branch officers find it a suitable retirement home.

“Retired police horses are not sold on and can be returned to the care of the MPS at any time. In 2008 a retired MPS horse was loaned to Rebekah Brooks. The horse was subsequently re-housed with a police officer in 2010."